The 28th International Cosmic Ray Conference

The Telescope Array Experiment:
Physics Aims

1An Overview andY.Arai,1 J.Beltz,2 M.Chikawa,3 H.Fujii,1 M.Fukushima,4 K.Hashimoto,5 Y.Hayashi,6N.Hayashida,4 K.Hibino,7 K.Honda,5 N.Inoue,8 C.Jui,9 K.Kadota,10 F.Kakimoto,11K.Kasahara,12 H.Kawai,13 S.Kawakami,6 K.Martens,9 T.Matsuda,1 T.Nakamura,14S.Ogio,11M.Ohnishi,4H.Ohoka,4N.Sakurai,4M.Sasano,15S.Schnetzer,1649417
1H.Shimodaira,P.Sokolsky, M.Takita,K.Tanaka,M.Tanaka, Y.Tanaka,18
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16
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M.Teshima,G.Thomson,R.Torii,S.Uchihori,
N.Yasuda20 S.Yoshida,13
H.Yoshii,21 T.Yoshikoshi,6(1) Institute of Particle and Nuclear Studies, KEK, Tsukuba 305-0801, Japan(2) Montana State University, Bozeman, MT 59717, USA(3) Kinki University, Osaka 577-8502, Japan(4) Institute for Cosmic Ray Research, University of Tokyo, Chiba 277-8582, Japan(5) Yamanashi University, Kofu 400-8510, Japan(6) Osaka City University, Osaka 558-8585, Japan(7) Kanagawa University, Yokohama 221-8686, Japan(8) Saitama University, Saitama 338-8570, Japan(9) University of Utah, Salt Lake City, Utah 84112, USA.
(10) Musashi Institute of Technology, Tokyo 158-8557, Japan(11) Tokyo Institute of Technology, Tokyo 152-8551, Japan(12) Shibaura Institute of Technology, Saitama 337-8570, Japan(13) Chiba University, Chiba 263-8522, Japan(14) Kochi University, Kochi 780-8520, Japan(15) Communications Research Laboratory, Tokyo 184-8795, Japan(16) Rutgers University, Piscataway, NJ 08854-8019, USA(17) Hiroshima City University, Hiroshima 731-3194, Japan(18) Nagasaki Institute of Applied Science, Nagasaki 851-01, Japan(19) Max Planck Institute for Physics, 80805 Muenchen, Germany(20) National Institute of Radiological Sciences, Chiba 263-8555, Japan(21) Ehime University, Matsuyama 790-8577, JapanAbstract
The Telescope Array (TA) experiment plans to deploy an array of 10 telescope stations in the west desert of Utah, USA and observes extremely high energy
cosmic rays (EHECRs) by the atmospheric ﬂuorescence. Its purpose is to study
super-GZK (E > 1020 eV) cosmic rays discovered by AGASA. In order to identify
the origin of super-GZK events, TA has ∼30 times larger aperture than AGASA
and provides a particle identiﬁcation by the shower proﬁle measurement. The ﬁrst
step of construction will start as a hybrid detector with an AGASA×10 groundarray and a part of ﬂuorescence telescopes, which will unambiguously establish
pp. 1–4c
2003
by Universal Academy Press, Inc.

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the existence of super-GZK and cluster events with much improved statistics andsystematics. This paper describes the physics of TA and its overall planning.
1.

Physics of Telescope Array

The AGASA so far observed 11 super-GZK cosmic rays in 12 years ofoperation. The energy spectrum continues toward high energy without showing
an indication of the GZK-cutoﬀ[1]. The arrival directions are isotropic and does
not show a correlation with the galactic plane or the super-galactic plane of nearby
galaxies. A few point-like clusters were observed in the sky, from which 2 or more
EHECRs were arriving[2]. High energy astronomical objects have been searchedbehind these events but no apparent candidates were found. A correlation withquasar remnants[3] and the BL Lac objects[4] had been suggested, but none seemdeﬁnitive at this moment. The latest results of AGAGA are reported elsewherein this conference. Several hypotheses have been proposed to explain super-GZK
events and clusters[5];• The production of EHECRs by the decay of long-lived, super-heavy relicparticles concentrated in the halo of our galaxy and with some level ofclumping to produce clustered events.
• The production of Z◦ by the collision of cosmological neutrino backgroundwith extremely high energy neutrinos (E≥ 1021 eV) produced deep in theuniverse. The EHECR is produced as a decay product of Z◦ . The primordialneutrinos may be concentrated in the local super-cluster of galaxies by its
gravitational interaction with the dark matter.
• The Lorentz invariance breaks down at a very high Lorentz factor suchthat the high interaction cross section of pion photoproduction via ∆(1232)
resonance is avoided.
• The generation of EHECRs by the yet unknown dark stars which are overpopulated by a factor of ∼10 in the vicinity of our galaxy.
Note that the Z-burst and the violation of Lorentz invariance explain the extensionof the spectrum above GZK cutoﬀ, but they leave the question of acceleration
itself unanswered. It is certain that super-GZK events and clusters are diﬃcult
to explain in the standard framework of astrophysics and particle physics.
2.

Prospects

The models with super-heavy relics and the Z-burst have a clear experimental signature to be tested, i.e. EHE gamma rays and neutrinos are an order ofmagnitude more abundant than protons in these models. The super-heavy relics

3Fig. 1. The Hybrid TA (left panel) is composed of 576 plastic scintillation counters
covering the ground area of 760 km2 and 3 fluorescence telescope stations (rightpanel) surrounding the ground array and looking inward.

model also expects a concentration of the sources in the center of the galaxy. Arealistic test of Lorentz invariance becomes possible when we identify the source
objects of EHECR and measure the distance to the source. The detection of heavy
nucleus as a major composition of the EHECR, on the other hand, excludes mostof the particle physics origin and directly leads us to the shock wave acceleration.
Thus, we expect the TA and the other next generation EHECR experiment will
give a clear answer to many of these hypotheses.
The experimental situation around the GZK cutoﬀ became unclear when
the HiRes group submitted a paper[6] recently on the ﬁrst measurement of theenergy spectrum and concluded that it is consistent with the existence of theGZK cut-oﬀ. Discussions after the submission of the paper can be summarized
as follows;• Taking a systematic uncertainty of the energy determination by two groups,
25% for HiRes[6] and 18% for AGASA[7], the claim of neither group on theexistence of the GZK cutoﬀ has a statistical signiﬁcance higher than 3 σ.
• Below 1020 eV where the comparison can be made with enough statistics, theenergy spectra of both agree perfectly well, if we remake the HiRes spectrum
by increasing the energy of all HiRes events by 25%, or by decreasing theAGASA energy by the same amount.
It is clear that the key of resolving the issue is the higher statistics and better
understanding of the systematics. This can be best achieved by a large hybrid
experiment making a simultaneous measurement with an AGASA type air shower

4Table 1.

Expected Acceptance, Resolution and Number of Events of Hybrid TA

Detector

Acceptance(km2 sr)
AGASA
162
1371
TA:Ground Array
TA:Fluorescence
670
165
TA:HybridAngularResolution
1.6◦
1.0◦
0.6◦
0.4◦

# of Events per Year
E > 1019 eV E > 1020 eV100
1700
9300
480
1array and a HiRes type air ﬂuorescence telescope. The good statistics can beobtained by building a large ground array, which is relatively inexpensive and its
stable operation and uniform coverage over the sky have been guaranteed by theexperience of AGASA and other arrays. The better understanding of systematics
can be made by the ﬂuorescence measurement, which allows us a calorimetricmeasurement of the shower energy, a direct observation of the arrival direction
and a determination of primary particle species by the shower longitudinal proﬁle.
We therefore propose to urgently build a hybrid TA, an AGASA×10ground array with a set of ﬂuorescence stations making a simultaneous measurement with the ground array (see Figure 1). It will be built in the West desertof Utah, USA, and will become a ﬁrst step of constructing the full TA[8]. The
performance of the hybrid TA detector is summarized in Table 1 and further details are given elsewhere in this conference. The plan is to complete the hybrid
TA by the end of 2006. We expect the full sky survey of EHECRs will be started
by the southern hemisphere Pierre Auger observatory in Argentina in 2005, andthe hybrid TA will start contributing to the northern hemisphere survey in 2007.
We hope that astonishing discoveries of AGASA are conﬁrmed, and the origin ofsuper-GZK and cluster is identiﬁed by 2010.
3.

References

1. Takeda, M. et al., 1998, Phys. Rev. Lett. 81, 1163
2. Chikawa, M. et al. 2001, Proceedings of 27th ICRC (Hamburg) HE1.4-3
3. Torres, D. F. et al. 2002, Phys. Rev. D66, 023001
4. Gorbunov, D. S. et al. 2002, Astrophys. J. 577, L93
5. Nagano, M., & Watson, A. A. 2000, Rev. Mod. Phys. 72, 689 and references therein
6. T.Abu-Zayyad et al. 2002, astro-ph0208301 and astro-ph0208243
7. Takeda, M. et al. 2002, astro-ph/0209422 submitted to Astropart. Phys.
8. The telescope Array Project: Design Report, July 2000